1. Field of the Invention
The present invention relates to a cooled turbine blade for a gas turbine, and more particularly to a full coveraged film cooling (FCFC) type turbine blade provided with film holes over the whole surface of the turbine blade.
2. Description of the Related Art
There are various types of gas turbines, and one of them is of a direct driving type for operating a compressor by means of a turbine driven by a burnt gas flow. In such a gas turbine, the higher the temperature of the burnt gas, i.e., the main flow gas is, the more the heat efficiency of the gas turbine is improved. Attempts have been made to increase the temperature of the main flow gas for this purpose. In general, however, the upper temperature of the main flow gas is limited by heat resistance of the turbine blade. In order to improve the heat resistance of the turbine blade, the turbine blade must withstand a high temperature. For the improvement of the heat resistance of the turbine blade, the heat resistance of the material of the turbine blade has been improved and the turbine blade has been cooled from its inside to lower its surface temperature. Such a turbine blade is provided in its interior with cooling flow passages through which a cooling gas such as water vapor or air passes and also provided with various other means for improving cooling efficiency. A cooling gas such as air is jetted out of a plurality of film holes formed in the blade surface. The blade surface is covered with a thin layer of the cooling gas, i.e., a film and cooled. The turbine blade is further provided with means for insulating heat from the main flow gas.
FIGS. 31 and 32 show a conventional cooled turbine blade of this type. FIG. 31 is a transverse cross-sectional view of the turbine blade and FIG. 32 is a longitudinal cross-sectional view thereof. The turbine blade 1 has an aerofoil blade portion 2 in which are formed a plurality of serially communicating cooling flow passages 4, 10, 12 and 13 extending in the span direction. The cooling gas flows through the cooling flow passages 12, 10 and 13 via passages formed in a shank 3 and cools wall portions 6 and 7 of the turbine blade. A plurality of nozzles 8 and 17 are formed in the wall portions 6 and 7. Part of the cooling gas flowing through the cooling flow passages 4 and 13 is jetted from the nozzles 8 and 17. The jetted cooling gas flows in a film state along the suction side surface and the pressure side surface of the aerofoil blade portion 2 so as to interrupt the heat transmitted to the surface of the aerofoil blade portion 2 and so as to cool the surface of the aerofoil blade portion 2. In this way, so-called film cooling is performed.
An impingement chamber is formed in the leading edge portion of the turbine blade 1. The cooling gas supplied to the cooling flow passage 4 is jetted out of a great number of holes and impinges on the inner surface of the wall 5 of the leading edge so as to perform so-called impingement cooling. In the wall 5 of the leading edge is formed a great number of nozzles so as to form a so-called shower head 9 from which the cooling gas in the impingement chamber or the leading edge chamber is jetted out to perform film cooling.
A trailing edge chamber is formed in the trailing edge portion 15 of the turbine blade 1. The cooling gas flows from the cooling flow passage 13 into the trailing edge chamber through a nozzle 14. In the trailing edge of the turbine blade 1 is formed a slit-like trailing edge nozzle 16 through which the cooling gas is exhausted from the trailing edge chamber to the outside thereof. A great number of pin fins 11 are formed in the trailing edge chamber and improves the cooling efficiency of the trailing edge portion 15.
In FIG. 33 is shown an insert impingement film type cooled turbine blade applied to the stator blade of a gas turbine. The cooled turbine blade 21 has a blade body 22 in which inserts 23a and 23b are inserted. The inner surface of the blade body is impinge cooled by a cooling gas 24 jetted from the inserts 23a and 23b. In the blade surface awe formed rows of film holes 25. The cooling gas jetted out of the cooling holes 25 film cools the turbine blade so that the turbine blade of the temperature is maintained to a predetermined value and thermal stresses produced in the turbine blade is reduced.
The interior of the inserts 23a and 23b are not partitioned, and the amount of flow of the cooling gas flowing in the turbine blade is suitably adjusted by a plurality of seal members disposed between the outer surface of the inserts and the blade body 22.
Since the thickness of the area of the turbine blade from its trailing edge portion 27 to its trailing edge 28 is small, this area does not have an enough room to receive such inserts. In place thereof, rows of pin fins 29 and a plurality of projecting turbulence promoters 30 are formed on the inner surfaces of the trailing edge portions 28. The cooling gas flows through the interior of the trailing edge 28 and is exhausted from openings 31 in the trailing edge 28.
In the turbine provided with the above-mentioned cooled turbine blade, an average surface temperature of the turbine blade of 850.degree. C. can be maintained when the temperature of the main flow is in the range between 1,000.degree. C. and 1,300.degree. C. and the amount of the cooling gas is several percent of the amount of the main flow gas. Recently, however, gas turbines operating at the temperature of the main flow gas from 1,300.degree. C. to 1,500.degree. C. have been developed, and further, development of gas turbines operating at the temperature of the main flow gas between 1,500.degree. C. and 2,000.degree. C. is now planned.
In the cooled turbine blade of the conventional gas turbine, the amount of the cooling gas must be extremely large in order to maintain the average surface temperature of 850.degree. C. The total heat efficiency of a gas turbine or the heat plant including this gas turbine is remarkably reduced and its actualization is very difficult.
In order to manufacture a practical gas turbine operating at a high temperature, the turbine blade must be designed so that a maximum cooling efficiency must be attained under the limited condition of the amount of the cooling gas as described above. When, however, the temperature of the main flow gas is extremely high, another new problem occurs in which the quantity of heat per unit area of the blade surface which flows on the blade surface increases. The quantity of heat transmitted through the material of the turbine blade per unit area increases and large thermal stresses are produced in the material.
The problem on production of the thermal stresses cannot be overcome even if the cooling efficiency achieved by improvement of the cooling gas is enhanced. If the surface of the temperature of the turbine blade is lowered by increasing the cooling efficiency due to the cooling gas conducted through the interior of the turbine blade, the difference between the surface temperature of the turbine blade and the temperature of the main flow gas, and the quantity of heat per unit area flowing on the blade surface adversely increases to elevate the thermal stresses. Increase of the quantity of heat per unit area and the accompanying thermal stresses shortens the life of the turbine blade. In a particular case of an electric power plant including gas turbines, they must be operated for a long rated time. Thus, the heat quantity per unit area and the accompanying thermal stresses cannot be increased.
In order to overcome such deficiencies, it is preferred that the film cooling effect of the blade surface be enhanced and the heat quantity per unit area flowing on the blade surface be reduced. By film cooling performed by a thin gas layer, i.e., a film flowing along the blade surface, the blade surface is cooled and the quantity of heat transmitted to the blade surface is reduced. Thus, the quantity of heat per unit area flowing on the blade surface is lowered. The cooling effect is increased in such a way that an FCFC turbine blade (a full coveraged film cooling type turbine blade) is used by increasing the number of the film holes.
The amount of the cooling gas required for film cooling the FCFC turbine blade becomes more than that of the conventional cooled turbine blade, and the total heat efficiency of the heat plant including a gas turbine is reduced.